PCM1740ESLASH2K datasheet(8/26 Pages) TI2

8 / 26 page

PCM1740

SBAS128B

www.ti.com

The 27MHz clock is available at the MCKO output (pin 5).

The VCXO output frequency can be precisely tuned using a

control voltage at the XTUN input (pin 3). The tuning range

is 27MHz

±150ppm typical for a 0V to +3V control voltage

range. Figure 3 shows the VCXO equivalent circuit, while

Figure 4 shows the typical tuning curve.

At power up, the VCXO requires 5ms start up time. The

VCXO also exhibits a 10

µs settling time in response to

changes in the XTUN control voltage. VCXO operation and

the MCKO output are not affected by the power on or

external reset functions, continuing to operate during the

initialization sequence.

Crystal Selection

The VCXO connects to an external 27MHz crystal via XT1

(pin 1) and XT2 (pin 24). The crystal should be AT-cut,

fundamental mode with

±30ppm accuracy and less than 50Ω

motional resistance. Crystal shunt capacitance should be 3pF

maximum, while load capacitance should be less than 7pF.

Miniature lead type or surface-mount devices are recom-

mended. External load capacitors are not needed, since they

are provided on-chip. The crystal should be placed as close as

possible to the XT1 and XT2 pins to reduce effects of parasitic

capacitance and land resistance.

PROGRAMMABLE PHASE LOCKED LOOP (PLL)

The PCM1740 includes an on-chip PLL for generating a 256fS

or 384fS audio system clock from the 27MHz VCXO output.

A block diagram of the PLL section is shown in Figure 5. The

PLL output clock is used by the digital filter and delta-sigma

modulator circuitry, and is made available at the SCKO output

(pin 21) for use with additional audio converters and signal

processors.

Tuning Voltage (V)

27.005

27.004

27.003

27.002

27.001

27.000

26.999

26.998

26.997

26.996

26.995

0.0

0.5

1.5

1.0

2.0

2.5

3.0

3.5

4.0

27MHz

Crystal

XTUN

0V to +3V

VCXO

MCKO

27MHz

±150ppm

SCKO

256/384f

Frequency

Selection

ROM

Phase

Detector

and

Loop Filter

PLL

N Counter

Frequency Selection

Control Register 3

VCO

M Counter

FIGURE 3. VCXO Equivalent Circuit.

FIGURE 4. VCXO Output Frequency (MCKO) versus

Tuning Voltage (XTUN).

XT1

27MHz

Crystal

27MHz

Tuned Clock

Voltage

Range

0 to 3V

XTUN

XT2

FIGURE 5. PLL Block Diagram.

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